Just to add, they usually withdraw the fluid from the cavity around the lungs using a long needle. I have never heard of this to remove fluid IN a lung.
I'll take it that I'm correct in the answer, that the fluid is in the pleural cavity around the lungs. The fluid is obviously built up so much that the lung cannot inflate properly, it will be squashed by the fluid. This causes breathing difficulty and a lot of discomfort. His body will not be able to remove that amount of fluid quickly enough and there is a possibility that it will increase. They can remove the fluid easily and if necessary, install a tiny drain so the procedure doesn't have to be repeated. I think he would be wise to have it done. I have actually watched this being done on a patient who was in a bed next to me a couple of years ago. They numbed the area with a general anaesthetic, and I was amazed, they withdrew 3 large syringes full of fluid. He was then breathing normally and out of pain. He felt absolutely nothing and I know this because I could see his face. He won't be able to see what is going on, they are around his side/back. Pleural effusion (fluid around the lungs) can be caused by surgery and once the fluid is removed, it's unlikely to return. However, I would suggest the drain to them if they feel there is a chance of more. I would also question the condition of his heart, ask if he has heart failure.
You have to be absolutely sure about this. Is the fluid IN his lung? or in the cavity SURROUNDING the lung? Both are very different and have very different causes.